What I don't get, is that if and when I compete an Ironman, I'm unlikely to need a reminder. I'll have been there at the time, never to be forgotten kind of thing. Seems like a creepy needy/self-esteem thing.

If you've competed in the Olympics then IMO that's ok in iteself (though going on about it all the time is a different matter...) - it's properly the pinnacle of (most) sporting achievement. On the otherhand an Ironman tattoo is a bit silly IMO - pretty much anyone could do an Ironman if they were so inclined. If you win one, that's a different story but crawling round in 17 hours isn't really a big deal in the scheme of things.